Arc Detection and Prevention in a Power Generation System

ABSTRACT

A method for arc detection in a system including a photovoltaic panel and a load connectible to the photovoltaic panel with a DC power line. The method measures power delivered to the load thereby producing a first measurement result of the power delivered to the load. Power produced by the photovoltaic panel is also measured, thereby producing a second measurement result of power produced by the photovoltaic panel. The first measurement result is compared with the second measurement result thereby producing a differential power measurement result. Upon the differential power measurement result being more than a threshold value, an alarm condition may also be set. The second measurement result may be modulated and transmitted over the DC power line.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.17/154,150, filed on Jan. 21, 2021, which is a continuation of U.S.application Ser. No. 15/479,530, filed on Apr. 5, 2017, which is acontinuation of U.S. application Ser. No. 13/290,528, filed on Nov. 7,2011, which claims priority to patent application GB1018872.0, filedNov. 9, 2010, in the United Kingdom Intellectual Property Office. Allthe prior applications are incorporated by reference in their entirety.

FIELD

The present invention is related to distributed power generation systemsand specifically to arc detection and prevention in photovoltaic powergeneration systems.

BACKGROUND

A distributed photovoltaic power generation system may be variouslyconfigured, for example, to incorporate one or more photovoltaic panelsmounted in a manner to receive sunlight such as on a roof of a building.An inverter may be connected to the photovoltaic panels. The invertertypically converts the direct current (DC) power from the photovoltaicpanels to alternating current (AC) power.

Arcing may occur in switches, circuit breakers, relay contacts, fusesand poor cable terminations. When a circuit is switched off or a badconnection occurs in a connector, an arc discharge may form across thecontacts of the connector. An arc discharge is an electrical breakdownof a gas, which produces an ongoing plasma discharge, resulting from acurrent flowing through a medium such as air, which is normallynon-conducting. At the beginning of a disconnection, the separationdistance between the two contacts is very small. As a result, thevoltage across the air gap between the contacts produces a very largeelectrical field in terms of volts per millimeter. This large electricalfield causes the ignition of an electrical arc between the two sides ofthe disconnection. If a circuit has enough current and voltage tosustain an arc, the arc can cause damage to equipment such as melting ofconductors, destruction of insulation, and fire. The zero crossing ofalternating current (AC) power systems may cause an arc not to reignite.A direct current system may be more prone to arcing than AC systemsbecause of the absence of zero crossing in DC power systems.

Electric arcing can have detrimental effects on electric powerdistribution systems and electronic equipment, and in particular,photovoltaic systems, which are often arranged in a manner thatincreases the risk of arching. For example, photovoltaic panels oftenoperate at extreme temperatures due to their necessary exposure to thesun. Such conditions cause accelerated deterioration in insulation andother equipment that can lead to exposed wires. Such systems are alsoexposed to environmental conditions, such as rain, snow, and highhumidity. Further, typical residential and/or industrial photovoltaicapplications often utilize several panels connected in series to producehigh voltage. Exposed conductors with high voltage in wet/humidconditions create an environment in which the probability of archingincreases.

This problem of arching raises system maintenance cost and reduces thelifespan of photovoltaic panels, because photovoltaic panels and otherrelated equipment will need to be repaired and/or replaced morefrequently. Arching in photovoltaic systems also increases the risk offire, thereby increasing operating and/or insurance cost on facilitieshaving photovoltaic systems. The net effect of arching in photovoltaicsystems is to increase the threshold at which a photovoltaic systembecomes cost competitive with nonrenewable sources of energy, such asnatural gas, oil, and coal.

BRIEF SUMMARY

As newly described herein, systems and methods are presented to addressthe problem of arching in photovoltaic systems, thereby reducing theoverall cost, and extending the useful lifespan of such systems. Theembodiments described herein, therefore, make deployment of photovoltaicsystems in residential and industrial application more competitive withnonrenewable energy alternatives.

Methods are provided for arc detection in a photovoltaic panel system,which may include a load connectible to the photovoltaic panel with oneor more mechanisms such as a power line, e.g. a DC power line. Anexemplary method may measure power delivered to the load and powerproduced by the photovoltaic panel. These measurements may be analyzedusing a suitable technique. One example of a suitable technique includesa comparison to generate, for example, a differential power measurementresult. The differential power measurement result may be furtheranalyzed using, for example, one or more static and/or dynamic thresholdvalues. The analysis may trigger, for example, an alarm condition whenthe differential power measurement results deviate from one or morethreshold values, either at an instant in time or over a time periodwhen the signal is integrated or smoothed. One or more of themeasurements (e.g., the second measurement), the static and/or dynamicthresholds, and/or the power measurements may be converted to a suitableformat and/or modulation scheme and transmitted to a remote location. Inone exemplary method, one or more of the foregoing items (e.g., thesecond measurement) may be modulated and transmitted (e.g., over the DCpower line) to a remote location.

According to further aspects, a device for arc detection in a system mayinclude a photovoltaic panel and a load connectible to the photovoltaicpanel using, for example, a power line (e.g. a DC power line). In thisaspect, the device may be variously configured to include one or moreelectronic modules adapted for measuring power produced by one or morephotovoltaic panels and/or a distributed and/or centralized controlleradapted for measuring power delivered to, for example, the load. Aspectsmay be variously configured to include one or more mechanisms to analyzepower associated with the photovoltaic panel and/or power delivered tothe load, dynamically and/or statically, in an instantaneous and/orintegrated manner. The analysis may be variously configured to include,for example, dynamic and/or static comparisons of an instantaneousand/or integrated signal. Suitable comparisons may or may not includeone or more thresholds. The analysis may collect historical data anddetermine variations from this historical data. Additionally, theanalysis may include predetermined threshold values based on prior testdata. Based on the dynamic and/or static comparison, one or more of themechanisms may be operable to detect arcing when the power output of oneor more photovoltaic panels is greater than the power delivered to theload.

According to further aspects, a method for arc detection may beperformed in a system having, for example, a photovoltaic string and aload connectible to the photovoltaic string using, for example, a DCpower line. The method for arc detection measurement may be variouslyconfigured, for example, to quantify a value associated with a noisevoltage of the load and/or a noise voltage of one or more of thephotovoltaic panels in the photovoltaic string. The quantitiesassociated with the various measured noise voltages may be analyzedusing a suitable technique. In one technique, a dynamic and/or staticcomparison is made between the various noise voltages e.g., (the noisevoltage of the load compared with the noise voltage of one or more (e.g.all) of the photovoltaic panels in the photovoltaic string) producing aquantitative value such as a differential noise voltage value(s). Thedifferential noise voltage value(s) may then be analyzed eitherstatically and/or dynamically. In one embodiment, the differential noisevoltage values(s) may be compared against one or more threshold values,statically and/or dynamically, instantaneously and/or integrated overtime and then compared. Where a threshold is utilized, an alarmcondition may be triggered where one or more of the aforementionedvalues exceed a threshold. For example, upon the differential noisevoltage result being more than a threshold value then an alarm conditionmay be set; upon the alarm condition being set, the photovoltaic stringmay be disconnected. The various parameters discussed above may beanalyzed locally and/or transmitted to a remote location. In oneembodiment, one or more of the values may be modulated and transmittedover a DC power line. Upon the power of one or more or all of thephotovoltaic panels or the power of photovoltaic string(s) being greaterthan the power as delivered to the load, then an alarm condition is setaccording to a previously defined static and/or dynamic criterion.

According to further aspects, one of the methods for arc detection mayinclude software and/or circuits for measuring power delivered to theload and/or power produced by the photovoltaic string

The measurement of the power of the photovoltaic string may be variouslyconfigured. In one example, the measurement involves sendinginstructions to measure the power output of each photovoltaic panel. Thepower value of each photovoltaic panel may then be transmitted andreceived. The power value of each photovoltaic panel may be added,thereby giving the second measurement result. The second measurementresult may then be subsequently modulated and transmitted over the DCpower line.

The load impedance may be changed according to a predetermined value.The power of the photovoltaic string, in this example, may then bemeasured again, thereby producing a third measurement result of thepower of the photovoltaic string. Followed by the power of the loadbeing measured, thereby producing a measurement result of the power ofthe load. The various measurements may be compared, thereby producinganother differential power result. The various differential powerresults may thereby produce a total differential power result. In thisexample, upon the total differential power result being more than athreshold value, an alarm condition may be set. Upon the alarm conditionbeing set, the photovoltaic string may be disconnected in the example.The third measurement result may be modulated and transmitted over theDC power line.

In this example, the measuring of the power of the photovoltaic stringmay involve sending one or more instruction to measure the power of eachphotovoltaic panel. The power value of each photovoltaic panel may thenbe transmitted and received. The power value of each photovoltaic panelmay be added, thereby giving the third measurement result. The thirdmeasurement result may then subsequently modulated and transmitted overthe DC power line.

In a further example, the sending of instructions to measure power inthe string may be to a master module connected to one of the panels ofthe string. Embodiments may also include slave modules respectivelyconnected to other panels of the string, which may be instructed tomeasure power. Power measurement results may then be transmitted fromthe slave modules to the master module. The power measurement resultsmay then be received by the master module, added up by the master moduleto produce a string power result, which may be transmitted to a centraland/or distributed controller in this example.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1a illustrates an example of circuit showing serial arcing.

FIG. 1b illustrates the circuit of FIG. 1a showing an example ofparallel or shunt arcing.

FIG. 2 shows a power generation system including an arc detectionfeature according to an embodiment of the present invention.

FIG. 3 shows a method according to an embodiment of the presentinvention.

FIG. 4 shows a method according to an embodiment of the presentinvention.

FIG. 5a shows a power generation circuit according to an embodiment ofthe present invention.

FIG. 5b shows a method according to an embodiment of the presentinvention.

FIG. 5c shows a method step shown in FIG. 5b , in greater detail, thestep measures power of a string according to an embodiment of thepresent invention.

FIG. 5d shows a method for serial arc detection, according to anotherembodiment of the present invention.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments, examples of whichare illustrated in the accompanying drawings, wherein like referencenumerals refer to the like elements throughout. The embodiments aredescribed below to explain the present invention by referring to thefigures.

Reference is made to FIG. 1a which shows serial arcing 106 in a circuit10 a according to background art. In FIG. 1a , a direct current (DC)power supply 102 provides power between power lines 104 a and 104 b.Power line 104 b is shown at ground potential. Load 100 connects powerline 104 b to power line 104 a. Serial arcing may occur in any part ofcircuit 10 a in power lines 104 a, 104 b or internally in load 100 orsupply 102 for example. A disconnection or poor connection in power line104 a between point C and point A is shown which causes an instance 106of serial arcing. Typically, if series arc 106 can be detected, circuitbreakers (not shown) located at supply 102 or load 100 can be tripped toprevent continuous serial arcing 106.

Reference is made to FIG. 1b , which shows parallel or shunt arcing 108in a circuit 10 b according to background art. In circuit 10 b, a directcurrent (DC) power supply 102 provides power between power lines 104 aand 104 b. Load 100 connects power lines 104 a and 104 b. Parallelarcing may occur in many parts of circuit 10 b, examples may includearcing between the positive of supply 102 and the ground/chassis ofsupply 102, if power supply cable 104 a/b is a two core cable; arcingmay occur between the two cores, or between the positive terminal 104 aand ground 104 b of load 100. Parallel arcing 108 may occur as shownbetween power line 104 b at point D and high potential on power line 104a at point C.

Arc noise is approximate to white noise, meaning that the power spectraldensity is nearly equal throughout the frequency spectrum. Additionally,the amplitude of the arc noise signal has very nearly a Gaussianprobability density function. The root mean square (RMS) arc noisevoltage signal (V_(n)) is given in equation Eq. 1, as follows:

V _(N)=√{square root over (4KTBR)}  Eq. 1,

-   -   where:    -   K=Boltzmann's constant=1.38×10⁻²³ Joules per Kelvin;    -   T=the temperature in degrees Kelvin;    -   B=bandwidth in Hertz (Hz) over which the noise voltage (V_(N))        is measured; and    -   R=resistance (ohms) of a resistor/circuit/load.

Reference is now made to FIG. 2, which shows a power generation system201 including an arc detection feature according to an embodiment. Aphotovoltaic panel 200 is preferable connected to an input of a module202. Multiple panels 200 and multiple modules 202 may be connectedtogether to form a serial string. The serial string may be formed byconnecting the outputs of modules 202 in series. Multiple serial stringsmay be connected in parallel across a load 250. Load 250 may be, forexample, a direct current (DC) to alternating current (AC) inverter orDC-to-DC converter. An electronic module 202 may be included to measuresthe voltage and/or current produced by a panel 200. Module 202 may becapable of indicating the power output of a panel 200. Attached to load250 may be a controller 204. Controller 204 may be operatively attachedto modules 202 via power line communications over DC power linesconnecting load 250 to the serial strings and/or by a wirelessconnection. Controller 204 may be configured to measure via sensor 206,the power received by load 250. Each panel 200 has a chassis, which maybe connected to ground. An instance of serial arcing 106 may occurbetween two panels 200. An instance of parallel arcing 108 may be shownbetween the positive terminal of a panel 200 and ground of the panel200.

Reference is now made to FIG. 3, which shows a method 301 according toone embodiment. Method 301 may be configured to detect serial and/orparallel arcing. Central controller 204 may be configured to measure oneor more parameters such as the power received by load 250 (step 300).Module 202 may be variously configured such as to measure the power ofone or more panels 200 (step 302). Module 202 may be variouslyconfigured. In one embodiment, it transmits a datum representing thepower measured of the one or more panels 200 via wireless or power linecommunications to controller 204. Controller 204 calculates thedifference between power generated at panel(s) 200 and the powerreceived at load 250 (step 304). In this example, if the differencecalculated in step 304 shows that the power generated at panel(s) 200may be greater than the power received at load 250 (step 306) accordingto a predefined criteria, an alarm condition of potential arcing may beset (step 308). Otherwise, in this example, the arc detection continueswith step 300.

Reference is now made to FIG. 4, which shows an illustrative method 401.Method 401 is a method, which may be utilized for detecting serialand/or parallel arcing. In a method according to this example, centralcontroller 204 measures (step 400) the root mean square (RMS) noisevoltage of load 250. Module 202 may then measure (step 402) the rootmean square (RMS) noise voltage of one or more panels 200. Module 202may be configured to transmit a datum representing the RMS noise voltagemeasured of panel(s) 200 via wireless or power line communications tocontroller 204.

One or more controllers may be configured to compare the noise voltageat panel(s) 200 with the noise voltage at the load 250 by, for example,calculating the difference between noise voltage measured at panel 200and the noise voltage measured at load 250 (step 404). In this example,if the difference calculated in step 404 shows that noise voltagemeasured at panel(s) 200 may be greater than the noise voltage measuredat load 250 (step 406) according to one or more predefined criteria, analarm condition of potential arcing may be set (step 408).

Further to this example, the comparison (step 404) also may involvecomparisons of previously stored RMS noise voltage levels of panel§ 200and/or load 250 in a memory of controller 204 at various times, forexample, the time immediately after installation of power generationsystem 201. The previously stored RMS noise voltage levels of bothpanel§ 200 and load 250 are, in this example, in the form of alook-up-table stored in the memory of controller 204. The look-up-tablehas RMS noise voltage levels of both panel(s) 200 and load 250 atvarious times of the day, day of the week or time of year for example,which can be compared to presently measured RMS noise voltage levels ofboth panel(s) 200 and load 250.

In this exemplary example, if the comparison of the measured load 250RMS noise voltage datum with the measured panel(s) 200 RMS noise voltagedatum may be over a certain threshold (step 406) of RMS noise voltagedifference an alarm condition of potential arcing may be set (step 408)otherwise arc detection continues with step 400.

Reference is now made to FIG. 5a which shows a power generation circuit501 a according to an embodiment of the present invention. Powergeneration circuits 501 a have outputs of panels 200 connected to theinput of modules 202. The outputs of panels 200 may be configured toprovide a DC power input (P_(IN)) to modules 202. Modules 202 mayinclude direct current (DC-to-DC) switching power converters such as abuck circuit, a boost circuit, a buck-boost circuit, configurablebuck-or-boost circuits, a cascaded buck and boost circuit withconfigurable bypasses to disable the buck or boost stages, or any otherDC-DC converter circuit. The output voltage of modules 202 may belabeled as V_(i).

The outputs of modules 202 and module 202 a may be connected in seriesto form a serial string 520. Two strings 520 may be shown connected inparallel. In one string 520, a situation is shown of an arc voltage(V_(A)) which may be occurring serially in string 520. Load 250 may be aDC to AC inverter. Attached to load 250 may be a central controller 204.Controller 204 optionally measures the voltage (V_(T)) across load 250as well as the current of load 250 via current sensor 206. Currentsensor 206 may be attached to controller 204 and coupled to the powerline connection of load 250.

Depending on the solar radiation on panels 200, in a first case, somemodules 202 may operate to convert power on the inputs to give fixedoutput voltages (V_(i)) and the output power of a module 202 that may bedependent on the current flowing in string 520. The current flowing instring 520 may be related to the level of irradiation of panels 200,e.g., the more irradiation, the more current in string 520, and theoutput power of a module 202 is more.

In a second case, modules 202 may be operating to convert powers on theinput to be the same powers on the output; so for example if 200 wattsis on the input of a module 202, module 202 may endeavor to have 200watts on the output. However, because modules 202 may be connectedserially in a string 520, the current flowing in string 520 may be thesame by virtue of Kirchhoff's law. The current flowing in string 520being the same means that the output voltage (V_(i)) of a module shouldvary in order to establish that the power on the output of a module 202may be the same as the power on the input of a module 202. Therefore, inthis example, as string 520 current increases, the output voltage(V_(i)) of modules 202 decreases or as string 520 current decreases, theoutput voltage (V_(i)) of modules 202 increases to a maximum value. Whenthe output voltage (V_(i)) of modules 202 increases to the maximumvalue, the second case may be similar to the first case in that theoutput voltage (V_(i)) may be now effectively fixed.

Modules 202 in string 520 may have a master/slave relationship with oneof modules 202 a configured as master and other modules 202 configuredas slaves.

Since current may be the same throughout string 520 in this example,master module may be configured to measure current of string 520.Modules 202 optionally measure their output voltage V_(i) so that thetotal string power may be determined. Output voltages of slave modules202, in this example, may be measured and communicated by wireless orover power line communications, for instance to master unit 202 a sothat a single telemetry from module 202 a to controller 204 may besufficient to communicate the output power of the string. Master module202 a in string 520 may be variously configured, such as to communicatewith the other slave modules 202 for control of slave modules 202.Master module 202 a, in this example, may be configured to receive a‘keep alive’ signal from controller 204, which may be conveyed to slavemodules 202. The optional ‘keep alive’ signal sent from controller 204communicated by wireless or over power line communications, may bepresent or absent. The presence of the ‘keep alive’ signal may cause thecontinued operation of modules 202 and/or via master module 202 a. Theabsence of the ‘keep alive’ signal may cause the ceasing of operation ofmodules 202 and/or via master module 202 a (i.e. current ceases to flowin string 520). Multiple ‘keep alive’ signals each having differentfrequencies corresponding to each string 520 may be used so that aspecific string 520 may be stopped from producing power where there maybe a case of arcing whilst other strings 520 continue to produce power.

Reference is now also made to FIG. 5b which shows a method 503 accordingto an embodiment of the present invention. In step 500, one or morestrings 520 power may be measured. In step 502, the load 250 power maybe measured using central controller 204 and sensor 206. The measuredload power and the measured string powers may be compared in step 504.Steps 500, 502 and 504 may be represented mathematically by Equation Eq.2 (assuming one string 520) with reference, in this example, to FIG. 5a, as follows:

V _(T) I _(L) =ΣP _(IN) −V _(A)[I _(L)]I _(L) +ΣV _(i) I _(L)  Eq. 2,

-   -   where:    -   V_(A)[I_(L)]=the arc voltage as a function of current I_(L);    -   V_(T) I_(L)=the power of load 250;    -   Σ P_(IN)=the power output of modules 202 when modules 202 may be        operating such that the output voltage (V_(i)) of a module        varies in order to establish that the power on the output of a        module 202 may be the same as the power on the input of a module        202 (P_(IN)); and    -   Σ V_(i) I_(L)=the power output of modules 202 with fixed voltage        outputs (V_(i)) and/or power output of modules 202 (with        variable output voltage V_(i)) when string 520 current decreases        sufficiently such that the output voltage (V_(i)) of modules 202        increases to a maximum output voltage level value. In all cases,        the maximum output voltage level value (V_(i)) and fixed voltage        outputs (V_(i)) may be pre-configured to be the same in power        generation circuit 501 a.

The comparison between string power of string 520 and of the power(V_(T)×I_(L)) delivered to load 250 may be achieved by subtracting thesum of the string 520 power (ΣP_(IN)+ΣV_(i) I_(L)) from the powerdelivered to load 250 (V_(T)×I_(L)) to produce a difference. If thedifference may be less than a pre-defined threshold (step 506), themeasurement of power available to string 520 (step 500) and load 250(step 502) continues. In decision block 506, if the difference may begreater than the previously defined threshold, then an alarm conditionmay be set and a series arc condition may be occurring. A situation ofseries arcing typically causes the transmission of a ‘keep alive’ signalto modules 202 from controller 204 to discontinue, which causes modules202 to shut down. Modules 202 shutting down may be a preferred way tostop series arcing in string 520.

Reference is now made to FIG. 5c which shows method step 500 (shown inFIG. 5b ) in greater detail to measure power of string 502 according toan embodiment of the present invention. Central controller 204 may sendinstructions (step 550) via power line communications to master module202 a. Master module 202 a may measure the string 502 current as well asvoltage on the output of master module 202 a and/or voltage and currenton the input of master module 202 a to give output power and input powerof module 202 a respectively. Master module may instruct (step 552)slave modules 202 in string 502 to measure the output voltage and string502 current and/or the input voltage and current of modules 202 to giveoutput power and input power of modules 202 respectively. Slave modules202 may then be configured to transmit (step 554) to master module 202 athe input and output powers measured in step 552. Master module 202 areceives (step 556) the transmitted power measurements made in step 554.Master module 202 a then adds up the received power measurements alongwith the power measurement made by master module 202 a (step 558)according to equation Eq. 2. According to equation Eq. 2; Σ P_(IN)=thepower output of modules 202 when modules 202 may be operating such thatthe output voltage (V_(i)) of a module varies in order to establish thatthe power on the output of a module 202 may be the same as the power onthe input of a module 202 (P_(IN)); ΣV_(i) I_(L)=the power output ofmodules 202 with fixed voltage outputs (V_(i)) and/or power output ofmodules 202 (with variable output voltage V_(i)) when a string 520current decreases sufficiently such that the output voltage (V_(i)) ofmodules 202 increases to a maximum output voltage level value. In allcases, the maximum output voltage level value (V_(i)) and fixed voltageoutputs (V_(i)) may be pre-configured to be the same in power generationcircuit 501 a. The added up power measurements in step 558 may be thentransmitted by master module 202 a to central controller 204 (step 560).

Reference is now made to FIG. 5d , which shows a method 505 for serialarc detection, according to another embodiment of the present invention.First differential power result 508 occurs in circuit 501 a, with loadcurrent I_(L) now labeled as current I₁ and with voltage V_(T) acrossload 250 (as shown in FIG. 5a ). First differential power result 508 maybe produced with reference to FIG. 5a and equation Eq. 3 (below) as aresult of performing method 503 (shown in FIG. 5c ). Eq. 3 is asfollows:

V _(T) I ₁ =ΣP _(IN) −V _(A)[I ₁]I ₁ +ΣV _(i) I ₁  Eq. 3,

-   -   where:    -   V_(A)[I₁]=the arc voltage as a function of current I₁;    -   V_(T) I₁=the power of load 250;    -   Σ P_(IN)=the power output of modules 202 when modules 202 may be        operating such that the output voltage (V_(i)) of a module        varies in order to establish that the power on the output of a        module 202 may be the same as the power on the input of a module        202 (P_(IN)); and    -   ΣV_(i) I_(L)=the power output of modules 202 with fixed voltage        outputs (V_(i)) and/or power output of modules 202 (with        variable output voltage V_(i)) when string 520 current decreases        sufficiently such that the output voltage (V_(i)) of modules 202        increases to a maximum output voltage level value. In all cases,        the maximum output voltage level value (V_(i)) and fixed voltage        outputs (V_(i)) may be pre-configured to be the same in power        generation circuit 501 a.

The impedance of load 250 may be adjusted (step 510) optionally undercontrol of central controller 204. Typically, if load 250 is aninverter, controller 204 adjusts the input impedance of load 250 byvariation of a control parameter of the inverter. A change in the inputimpedance of load 250 causes the voltage across the input of load 250 tochange by virtue of Ohm's law. The voltage (V_(T)) as shown in circuit501 a across load 250 may be therefore made to vary an amount ΔV as aresult of the input impedance of load 250 being adjusted. The voltageacross load 250 may be now V_(T)+ΔV and the load 250 current (I_(L)) maybe now I₂.

A second differential power result 522 may be now produced as a resultof performing again method 503 (shown in FIG. 5c ) on the adjusted inputimpedance of load 250 performed in step 510. Second differential powerresult 522 may be represented mathematically by equation Eq. 4, asfollows:

(V _(T) +ΔV)I ₂ =ΣP _(IN) −V _(A)[I ₂]I ₂ +ΣV _(i) I ₂  Eq. 4,

-   -   where:    -   V_(A) [I₂]=the arc voltage as a function of current I₂;    -   (V_(T)+ΔV)I₂=the power delivered to load 250;    -   ΣP_(IN)=the power output of modules 202 when modules 202 may be        operating such that the output voltage (V_(i)) of a module        varies in order to establish that the power on the output of a        module 202 may be the same as the power on the input of a module        202 (P_(IN)); and    -   ΣV_(i) I_(L)=the power output of modules 202 with fixed voltage        outputs (V_(i)) and/or power output of modules 202 (with        variable output voltage V_(i)) when string 520 current decreases        sufficiently such that the output voltage (V_(i)) of modules 202        increases to a maximum output voltage level value. In all cases,        the maximum output voltage level value (V_(i)) and fixed voltage        outputs (V_(i)) may be pre-configured to be the same in power        generation circuit 501 a.

The first differential power result 508 may be compared with the seconddifferential power result 522 (step 524), for example, using controller204 to subtract the first differential power result 508 from the seconddifferential power result 522 to produce a difference. The differencemay be expressed by equation Eq. 5, which may be as a result ofsubtracting equation Eq. 3 from equation Eq. 4, as follows:

V _(T) I ₁−(V _(T) +ΔV)I ₂ =V _(A)[I ₂]I ₂ −V _(A)[I ₁]I ₁ +ΣV _(i)(I₁-I ₂)  Eq. 5

The summed output power (P_(IN)) of each module 202 for circuit 501 amay be thus eliminated.

Equation Eq. 5 may be re-arranged by controller 204 by performing amodulo operator function on equation Eq. 5 to obtain an arc coefficientα as shown in equation Eq. 6.

$\begin{matrix}{\frac{{V_{T}I_{1}} - {\left( {V_{T} + {\Delta V}} \right)I_{2}}}{\left( {I_{1} - I_{2}} \right)} = {\alpha + {\sum V_{i}}}} & {{Eq}.6}\end{matrix}$

-   -   where the arc coefficient α is shown in Eq. 7

$\begin{matrix}{\alpha = \frac{{{V_{A}\left\lbrack I_{2} \right\rbrack}I_{2}} - {{V_{A}\left\lbrack I_{1} \right\rbrack}I_{1}}}{\left( {I_{1} - I_{2}} \right)}} & {{Eq}.7}\end{matrix}$

Controller 204, for example, may be configured to calculate coefficientα according to the above formula and measurements. A non-zero value ofarc coefficient α shown in equation Eq. 7 causes an alarm condition tobe set (step 528) otherwise another first differential power result 508may be produced (step 503). A situation of series arcing typicallycauses the ‘keep alive’ signal to be removed by controller 204, causingmodules 202 to shut down. Modules 202 shutting down may be a preferredway to stop series arcing in string 520.

The definite articles “a”, “an” is used herein, such as “an arc voltageand/or arc current”, “a load” have the meaning of “one or more” that is“one or more arc voltages and/or arc currents” or “one or more loads”.

While the embodiments of aspects of the invention has been describedwith respect to a limited number of examples, it will be appreciatedthat many variations, modifications and other applications of theinvention may be made.

1. A system comprising: a first photovoltaic string configured toproduce a first string power, wherein the first photovoltaic stringcomprises a first plurality of photovoltaic panels; a module configuredto determine the first string power to produce a measured string power;a load configured to be connected to the first photovoltaic string andreceive the first string power from the first photovoltaic string; acontroller configured to be connected to the load and configured to:transmit a first signal to the first photovoltaic string, wherein thefirst signal comprises an instruction to continue power production bythe first photovoltaic string; measure a total power delivered to theload; receive the measured string power; compare the total powerdelivered to the load and the measured string power to produce a powerdifference; determine that the power difference is greater than athreshold value; and cease transmission of the first signal to the firstphotovoltaic string to stop power production of the first string power.2. The system of claim 1, wherein the module is configured to determinethe first string power by adding a first panel power and a second panelpower.
 3. The system of claim 1, further comprising a secondphotovoltaic string configured to produce a second string power, whereinthe second photovoltaic string comprises a second plurality ofphotovoltaic panels configured to provide the second string power to theload.
 4. The system of claim 3, wherein the controller is configured totransmit a plurality of signals, wherein the plurality of signalscomprises the first signal and a second signal, wherein the secondsignal comprises an instruction to continue power production by thesecond photovoltaic string, wherein the controller is configured totransmit the plurality of signals by power line communications, whereinthe first signal has a first frequency corresponding to the firstphotovoltaic string, and wherein the second signal has a secondfrequency corresponding to the second photovoltaic string.
 5. The systemof claim 4, wherein the controller is configured to transmit the secondsignal to instruct the second photovoltaic string to continue powerproduction of the second string power after transmission of the firstsignal has been ceased to stop power production of the first stringpower.
 6. The system of claim 5, wherein the controller is configured totransmit the second signal periodically.
 7. The system of claim 1,wherein the controller is configured to transmit the first signal to thefirst photovoltaic string by wireless communications.
 8. A methodcomprising: transmitting, using a controller, a first signal to a firstplurality of photovoltaic panels, wherein the first signal comprises aninstruction to continue power production by the first plurality ofphotovoltaic panels; measuring, using the controller, power delivered toa load; receiving, using the controller, a determined string powerproduced by the first plurality of photovoltaic panels; comparing, usingthe controller, the power delivered to the load and the determinedstring power to produce a power difference; determining, using thecontroller, that the power difference is greater than a threshold value;and ceasing, using the controller, transmission of the first signal tothe first plurality of photovoltaic panels to stop power production bythe first plurality of photovoltaic panels.
 9. The method of claim 8,further comprising determining the determined string power by adding afirst panel power and a second panel power.
 10. The method of claim 8,further comprising transmitting, using the controller, a second signal,wherein the second signal comprises an instruction to continue powerproduction by a second plurality of photovoltaic panels.
 11. The methodof claim 10, wherein the controller is configured to transmit the firstsignal and the second signal by power line communications, wherein thefirst signal has a first frequency corresponding to the first pluralityof photovoltaic panels, and wherein the second signal has a secondfrequency corresponding to the second plurality of photovoltaic panels.12. The method of claim 10, further comprising transmitting, using thecontroller, the second signal to instruct the second plurality ofphotovoltaic panels to continue power production by the second pluralityof photovoltaic panels after ceasing transmission of the first signal tostop power production by the first plurality of photovoltaic panels. 13.The method of claim 8, further comprising transmitting, using thecontroller, the first signal to the first plurality of photovoltaicpanels by wireless communications.
 14. An apparatus comprising: acontroller configured to: transmit a first signal to a first pluralityof photovoltaic panels, wherein the first signal comprises aninstruction to continue power production by the first plurality ofphotovoltaic panels; measure power delivered to a load; receive adetermined string power produced by the first plurality of photovoltaicpanels; compare the power delivered to the load and the determinedstring power to produce a power difference; determine that the powerdifference is greater than a threshold value; and cease transmission ofthe first signal to the first plurality of photovoltaic panels to stoppower production by the first plurality of photovoltaic panels.
 15. Theapparatus of claim 14, wherein the determined string power is determinedby adding a first panel power and a second panel power.
 16. Theapparatus of claim 14, wherein the controller is further configured totransmit a second signal, wherein the second signal comprises aninstruction to continue power production by a second plurality ofphotovoltaic panels.
 17. The apparatus of claim 16, wherein thecontroller is configured to transmit the first signal and the secondsignal by power line communications, wherein the first signal has afirst frequency corresponding to the first plurality of photovoltaicpanels, and wherein the second signal has a second frequencycorresponding to the second plurality of photovoltaic panels.
 18. Theapparatus of claim 16, wherein the controller is configured to transmitthe second signal to instruct the second plurality of photovoltaicpanels to continue power production by the second plurality ofphotovoltaic panels after transmission of the first signal has beenceased to stop power production by the first plurality of photovoltaicpanels.
 19. The apparatus of claim 16, wherein the controller isconfigured to transmit the second signal periodically.
 20. The apparatusof claim 14, wherein the controller is configured to transmit the firstsignal to the first plurality of photovoltaic panels by wirelesscommunications.